This invention relates to radioactivity well logging techniques and more particularly to such techniques which are applicable for determining the lateral flow speed of fluid in earth formations in the vicinity of a well borehole by the employment of neutron activation and the analysis of resultant gamma radiation produced thereby.
In secondary and tertiary recovery for petroleum deposits many of the recovery techniques employ the injection of water or chemical solutions into the earth formations comprising the reservoir from injection wells. In planning the recovery operation, the injection of water or chemical has in the past been limited by certain assumptions and/or approximations concerning the mobility of fluids in the formation comprising the reservoir. A crucial factor in such fluid injection programs is the vertical conformity of producing formation as well as its horizontal permeability and uniformity. In some reservoirs, formation lensing or horizontal partitioning by permeability barriers such as faults can occur. In such instances, apparently correlevant intervals of permeability may be separated from one well to another in the field by such formation lensing or permeability barriers being interposed across the interval of formation between the wells.
It is therefore apparent that some knowledge of the existence of such lensing or permeability barriers could prevent the injection of large amounts of costly chemicals or water into a recovery pattern in which formation continuity between injection wells of a field is partially or totally absent. This could prevent the expenditure of large sums of money, time and effort in a fruitless project to recover secondary or tertiary problem deposits.
A second application of the detection of lateral water-flow measurement is the mapping of the total flow throughout a petroleum reservoir to help in the operational planning of injecting chemicals or water and to assist in determining optimum withdrawal rates. Moreover, a knowledge of the lateral water flow characteristics of a particular formation in a producing field can help greatly in general understanding of the reservoir dynamics of the particular reservoir being produced.
Also it is sometime desirable in a reservoir with multiple producing intervals for a reservoir engineer to be able to delineate those producing zones which provide the most water influx or water drive to the production of petroleum. The mapping of lateral water movement in all zones both above and below the expected water table in the producing formation should supply this information to the reservoir engineer.
In the past reservoir engineers have been provided with relatively few and often inaccurate well logging instrumentation in order to determine the vertical conformance characteristics of the earth formations comprising a reservoir. This has led to resultant confusion as to the properties of the earth formations making up the reservoir. Radioactive tracer studies of the movement of fluids in the vicinity of a well borehole can be misleading in this respect because of the lack of uniform absorption of the tracer element into the flowing stream of formation water. Also, it is difficult to provide tracer isotopes with sufficient half life to be injected at an injection well and observe their movement days or even weeks later at a monitoring or producing well, in order to obtain some idea of the lateral flow speed or velocity of fluids in the formation comprising the reservoir.
Accordingly, it is a principal object of the present invention to provide a relatively economical and accurate technique for observing the lateral movement and later flow speed of formation fluids in the vicinity of a well borehole.
According to the technique of the present invention the relatively slow lateral movement of fluids in the vicinity of well borehole may be accurately determined by the neutron activation of the element sodium comprising a portion of the fluids present in the formation in the vicinity of the well borehole. The observation of the counting rate of decay of the radioactive isotope of sodium produced by the neutron activation as a function of time may then, according to the techniques of the present invention, be interpreted in terms of the flow speed of the earth formation fluids in the vicinity of the borehole. Techniques are provided in making this measurement for eliminating or substantially reducing the effects of background radiation due to other elements which are activated in the neutron activation process. By use of the novel techniques of the invention more accurate results than obtainable from previous radioactive tracer injection techniques due to the non-uniform acceptance of tracer materials by the formation are eliminated by the in-situ creation of the radioactive tracer element.